The present invention relates to a negative-working photosensitive composition or, more particularly, to a negative-working photosensitive composition sensitive to ultraviolet and far ultraviolet light and suitable for forming a finely patterned photoresist layer having excellent heat resistance against heat and dry etching.
As is well known, the process of fine patterning in the manufacture of semiconductor devices such as integrated circuits involves the techniques of photolithography, in which a semiconductor silicon wafer having a surface film of an oxide or nitride is first coated with a solution of a photoresist composition followed by drying to form a photoresist film thereon. Thereafter, a photomask bearing a desired pattern is laid on the photoresist film which is exposed to actinic rays through the photomask followed by a treatment of developing to form a finely patterned photoresist layer. The surface film of oxide or nitride in the thus uncovered areas is then subjected to etching to have the underlying surface of silicon exposed bare which is doped with a dopant by diffusion after removal of the photoresist layer. Semiconductor devices are finished by repeating the above described procedures to form a desired electric circuit with adjunction of necessary parts such as electrodes.
The photoresist materials used in the above described photolithographic process are classified into positive-working ones, of which the solubility in a developer solution is increased by exposure to actinic rays so that the photoresist layer is dissolved away by the developer solution in the exposed areas, and negative-working ones, of which the solubility in a developer solution is decreased by exposure to actinic rays so that the photoresist layer is dissolved away by the developer solution in the unexposed areas. A typical positive-working photoresist material is a photosensitive composition comprising a novolac resin and an o-quinone diazide compound and typical negative-working photoresist materials include cyclized rubber-based photosensitive compositions comprising a cyclized rubber as the polymeric base ingredient admixed with a photosensitive bisazide compound and poly(vinyl cinnamate)-based photosensitive compositions prepared from polyvinyl alcohol and cinnamyl chloride.
Turning now to the procedure of etching in the processing of semiconductor devices, it is a trend in recent years that the conventional wet-etching process using an etching solution is increasingly being replaced with the dry-etching process utilizing gas plasma in view of several advantages over the wet process. A problem in the dry-etching process, however, is the poor resistance of the negative-working photoresist materials against the attack of the gas plasma in the etching in comparison with the positive-working ones and none of the conventional negative-working photoresist materials can give quite satisfactory results. Although many of the negative-working photoresist materials are superior to positive-working ones in respect of the heat resistance, the heat resistance of conventionally available negative-working photoresist materials is still insufficient to comply with the processing conditions in the modern manufacturing process of semiconductor devices.